Power amplifier

ABSTRACT

A power amplifier amplifies an input signal having a fundamental frequency of which band width ranges between a first fundamental frequency F 1  and a second fundamental frequency F 2 . The power amplifier includes a power amplifier transistor for amplifying the input signal and an output matching circuit for suppressing a harmonic component included in an output signal from the power amplifier transistor. The output matching circuit includes: a first second-order harmonic series resonant circuit including a first inductor and a first capacitor and having a frequency twice as large as F 1  as a resonance frequency; and a second second-order harmonic series resonant circuit including a second inductor and a second capacitor and having a frequency twice as large as F 2  as a resonance frequency.

FIELD OF THE INVENTION

The present invention relates to a circuit technology for semiconductorpower amplifiers used in signal transmitting sections of wirelessportable terminals using radio frequency bands, such as mobile phones,and particularly relates to a technology for attaining broadbandtransmission frequency.

BACKGROUND ART

In recent years, multi-band and broadband use is progressing for want offrequency bands in the field of mobile phones, and therefore, poweramplifiers used in the mobile phones are demanded to have increasedpower added efficiency over a broadband range, which is an essentialcharacteristic.

The power added efficiency of a power amplifier depends dominantly andlargely on the characteristics of an output matching circuit of thepower amplifier. A semiconductor amplifier at the final stage of thepower amplifier performs nonlinear operation in high power output, andtherefore, circuits for processing harmonic signals generated asnonlinear components are the key. Particularly, a circuit for processinga second-order harmonic as a main harmonic is the key. In view of this,in general, a resonant circuit, in which a resonance frequency ismatched to the frequency of a second-order harmonic, is connected to anoutput circuit, and a target harmonic is short-circuited oropen-circuited to suppress excessive increase in output level of thesecond-order harmonic. The resonant circuit suppresses only thesecond-order harmonic in the entirety of an amplified output signal andis called, therefore, a second-order harmonic trap circuit in general.Further, the second-order harmonic is short-circuited generally in sucha manner that a trap line or an inductor component having an inductanceof 1 nH or larger is connected in series to a capacitor to form aresonant circuit having a second-order harmonic frequency as a resonancefrequency and the resonant circuit is shunt-connected to the outputcircuit.

Patent Document 1: Japanese Patent Application Laid Open Publication No.2002-43873

Patent Document 2: Japanese Patent Application Laid Open Publication No.2000-40928

SUMMARY OF THE INVENTION

The frequency band of the second-order harmonic is, however, twice aslarge as the frequency band of the fundamental frequency of a signalinput to the power amplifier, which involves, in association with anincrease in band width of the fundamental frequency, difficulty inconventional impedance control of the broadband second-order harmonic.Hence, it becomes difficult for a single output matching circuit toperform matching over a broadband range.

For tackling the above problems, there have been proposed a method forattaining a broadband use by employing a power amplifier composed ofpower amplifiers optimized for different frequencies (see PatentDocument 1), and a method for attaining broadband use by employing acomplicated circuit which switches a usable circuit block for eachfrequency (see Patent Document 2). The above conventional methods,however, makes the circuit complicated to increase the number ofcomponents, thereby increasing the costs.

In view of the foregoing, the present invention has its object ofenabling impedance control of a second-order harmonic of which bandwidth is wider than that of a fundamental frequency while avoiding acircuit being complicated.

To attain the above object, a power amplifier in accordance with thepresent invention uses a single output matching circuit for performingmatching over a broadband range. Specifically, means for suppressing thebroadband second-order harmonic is attained by providing, for processinga broadband second-order harmonic, a conventional second-order harmonictrap circuit (first second-order harmonic trap circuit) in which aninductor having an inductance of 1 nH or larger is connected in seriesto a capacitor and a second second-order harmonic trap circuit which iscomposed of an inductor having an inductance of 1 nH or lower and alarge capacitor and has a resonance frequency different from the firstsecond-order harmonic trap circuit. Wherein, the resonance frequency ofthe conventional first second-order harmonic trap circuit is matched tothe low frequency of the broadband second-order harmonic frequency, andthe resonance frequency of the second second-order harmonic trap circuitis matched to the radio frequency of the broadband second-order harmonicfrequency.

Description will be given to the operation principal of the poweramplifier in accordance with the present invention. Referring to aseries resonant circuit, the resonance frequency thereof is determinedby a product of the inductance and the capacitance in the circuit. Evenwhen the product is fixed for keeping the resonance frequency constant,however, the height and the width of the resonance vary according to theinductance and the capacitance. Namely, the resonance height and theresonance width increase as the inductor becomes small while thecapacitance becomes large. In reverse, the resonance height and theresonance width decreases as the inductance becomes large while thecapacitance becomes small. The conventional harmonic trap circuit fallsin the latter case, and the resonance width is set small. Because: wideresonance width involves adverse influence on the fundamental frequencycomponent, thereby causing loss of the fundamental frequency component.

FIG. 4 shows a result of suppression effect obtained in the case where aseries resonant circuit having a second-order harmonic frequency of 1.6GHz (the fundamental frequency is 800 MHz) as a resonance frequency isshunt-connected to a power amplifier. In FIG. 4, the graph 401 indicatesa result obtained in a case using a series resonant circuit having incombination a large inductance L of 1 nH or larger (1.2 nH) and a smallcapacitance C (8.2 pF), and the graph 402 indicates a result obtained ina case using a series resonant circuit having in combination a smallinductance L of 1 nH or lower (0.7 nH) and a large capacitance C (15pF). As shown in FIG. 4, comparison of the result of the graph 401 withthe result of the graph 402 shows approximately 5 dB decrease insuppression at 1.6 GHz harmonic frequency and approximately 4 dBincrease in band-pass characteristic at the fundamental frequency of 800MHz.

As described above, when two conventional harmonic trap circuits areprovided merely, the power amplifier exhibits excellent frequencyselectivity with a narrow resonance width. The narrow resonance width,however, results in insufficient suppression of the broadbandsecond-order harmonic. In contrast, when two harmonic trap circuithaving wider resonance width and high resonance height, that is, twoseries resonant circuit having small inductance and large capacitanceare provided for granting priority to suppression of the broadbandsecond-order harmonic, the second-order harmonic can be suppressedsufficiently while the wide resonance width influences adversely thefundamental frequency component to reduce the gain of the fundamentalfrequency.

In view of the foregoing, the present invention provides a firstsecond-order harmonic trap circuit designed to be matched to theresonance frequency corresponding to the low (lower limit) frequency F1of the broadband fundamental frequency and having large inductance andsmall capacitance, thereby attaining an object of suppressing thesecond-order harmonic while reducing the resonance width to prevent thefundamental frequency component from receiving the influence of a narrowresonance width. The present invention additionally provides a secondsecond-order harmonic trap circuit designed to be matched to theresonance frequency corresponding to the high (upper limit) frequency F2of the broadband fundamental frequency and having small inductance andlarge capacitance, thereby repressing the band-pass characteristic ofthe broadband second-order harmonic including the second-order harmonicof F1. In so doing, even with the broadband fundamental frequency, theinfluence on the fundamental frequency component is less than the casewhere the resonance frequency is matched to the second-order harmonic ofthe low frequency F1 because the resonance frequency is matched to theradio frequency F2 of the fundamental frequency.

In the present invention, the combination of the two kinds of harmonictrap circuits enables impedance control of the broadband second-orderharmonic.

Referring to the actual construction of the power amplifier of thepresent invention, the conventional technique can attain easily thefirst second-order harmonic trap circuit using F1 as a resonancefrequency and having large inductance and small capacitance. While, inattaining the second second-order harmonic trap circuit using F2 as aresonance frequency and having small inductance and large capacitance,some scheme is required for precisely providing a small inductance of 1nH or lower. In the present invention, a shunt C (branch point) isformed in the middle (on the transistor output end side) of a bias lineof an output transistor (a power amplifier transistor) to utilize ashort-distance part of the bias line from the output end of thetransistor to the shunt C as an inductor, thereby precisely providing asmall inductance of 1 nH or lower.

As described above, in the present invention, impedance of the broadbandsecond-order harmonic can be controlled by a single output matchingcircuit to attain highly-efficient and low-cost power amplifier with asimple circuit configuration.

In sum, the present invention provides a technique achieving high powerefficiency over a broadband range by a single power amplifier andattains a low-cost broadband power amplifier. Hence, the presentinvention is applicable to technology for a broadband power amplifier ofwhich usable field will expand in overall wireless communication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a power amplifier in accordance with oneembodiment of the present invention.

FIG. 2 is a graph showing a band-pass characteristic of an outputmatching circuit of the power amplifier in accordance with theembodiment of the present invention in comparison with a band-passcharacteristic of an output matching circuit of a power amplifier inaccordance with a comparative example.

FIG. 3 is a circuit diagram of the power amplifier in accordance withthe comparative example.

FIG. 4 is a graph showing a relationship between the band-passcharacteristic of an output matching circuit of an amplifier and theinductance or capacitance of a second-order harmonic trap circuit usedin the output matching circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A power amplifier in accordance with one embodiment of the presentinvention will be described below with reference to the accompanyingdrawings. It should be noted that the fundamental frequency of an inputsignal that the power amplifier of the present embodiment is to processis not limited specifically only if the fundamental frequency has abroadband width of 50 MHz or larger from a first fundamental frequencyF1 to a second fundamental frequency F2. The following descriptionrefers to a fundamental frequency having a broadband width between 800MHz and 900 MHz, both inclusive. In the present embodiment, accordingly,the first fundamental frequency F1 is 800 MHz, and the secondfundamental frequency F2 is 900 MHz.

FIG. 1 is a circuit diagram of the power amplifier of the presentembodiment which includes an output matching circuit of the presentinvention. Though not shown in FIG. 1, for an actual construction of thepower amplifier, there are provided a bias circuit of an output stagetransistor 102, a driver stage circuit up to a signal input terminal 100of the output stage transistor 102, a matching circuit for matching thedriver stage circuit with the output stage transistor 102, and the like.Namely, a signal from the driver stage circuit is input to the signalinput terminal 100 of the output stage transistor 102.

As shown in FIG. 1, bias voltage is supplied to the output stagetransistor 102 from a voltage source 103 via a bias line inductor 104.The bias inductor 104 has an inductance of, for example, 7 to 15 nH.While, the electric length of the bias inductor 104, which correspondsto an electric length of a part from a bypass capacitor 110 describedlater to a branch point of a shunt capacitor 109C described later, ispreferable to be λ/4, namely, open-circuited ideally, but may be in arange between λ/16 and λ/8, both inclusive in the actual construction.Wherein, λ represents a wavelength of an input signal. The bypasscapacitor 110 having a capacitance of, for example, 1000 pF or larger isprovided between the voltage source 103 and the bias line inductor 104.

On an output line from the output stage transistor 102 to an outputterminal 101 of the power amplifier, there are provided a seriesinductor (output line inductor) 105 having an inductance of, forexample, 2 to 3 nH, an output matching shunt capacitor 106 having acapacitance of, for example, 6 to 10 pF, and a output matching seriescapacitor 107 having a capacitance of, for example, 100 pF or larger.Thus, the fundamental frequency (primary frequency) component in therange between 800 MHz and 900 MHz is matched for the purpose ofbroadband amplification.

Further, in the present embodiment, two kinds of second-order harmonictrap circuits (second-order harmonic resonant circuits) of the presentinvention are provided, which will be described below in detail.

First, a first second-order resonant circuit 108 in which an inductor108L having a inductance of 1 nH or larger (for example, 1 to 2 nH) anda capacitor 108C having a capacitance of, for example, 6 to 9 pF areconnected in series to each other is provided at a general position,specifically, on a line branching from the output line between theoutput end of the output stage transistor 102 and the series inductor105. The capacitor 108C side end of the first second-order seriesresonant circuit 108 is grounded. The resonance frequency of the firstsecond-order series resonant circuit 108 is set to the second-orderharmonic of the first fundamental frequency F1 (800 MHz).

A shunt capacitor 109C having a capacitance (9 to 13 pF, for example)larger than the capacitor 108C is provided on a line branching in themiddle of the bias line, specifically, the line branching from the biasline at a predetermined branch point between the output end of theoutput stage transistor 102 and the bias line inductor 104. The oppositeend of the shunt capacitor 109C to the bias line is grounded. With theabove arrangement, a part of the bias line which ranges from the outputend of the output stage transistor 102 to the junction point (the abovepredetermined branch point) of the shunt capacitor 109C serves as aninductor 109L having an inductance of 1 nH or lower (0.5 to 1 nH, forexample). The output end of the output stage transistor 102 correspondsto a collector terminal when the output stage transistor 102 is abipolar transistor or a drain terminal when it is a FET (field-effecttransistor). In the present embodiment, a series resonant circuitcomposed of the inductor 109L and the shunt capacitor 109C functions asa second second-order series resonant circuit 109. The resonancefrequency of the second second-order resonant circuit 109 is set to thesecond-order harmonic of the second fundamental frequency F2 (900 MHz).

It is noted that the inductor 109L may be composed of a microstrip line,for example. Further, in the case, for example, where a semiconductorsubstrate on which the output stage transistor 102 is integrated ismounted in a package, the capacitor 108C may be integrated also on thesemiconductor substrate while a part from the output end of the outputstage transistor 102 to the end of the bias line, specifically, thevoltage source 103, the bypass capacitor 110, the bias line inductor104, the inductor 109L, and the shunt capacitor 109C may be componentsaccommodated in the package, namely, outside chip components notintegrated on the semiconductor substrate and inductors utilizing thestrip line in the package. In the case where the shunt capacitance 109Cis such a chip component, there is no need to connect the shuntcapacitance 109C to the predetermined branch point by wire bonding incontrast to the case where the shunt capacitance 109C is integrated onthe semiconductor substrate. This eliminates the need to provide aninductance of 1 nH or larger, facilitating attainment of broadband use.The shunt capacitance 109C may be integrated on another semiconductorsubstrate different from the semiconductor substrate on which the outputstage transistor 102 is integrated. This case corresponds to claim 4.

FIG. 2 shows comparison between the band-pass characteristic obtained inthe case where each series resonant circuit of the present invention isshunt-connected to the power amplifier and that obtained in the casewhere series resonant circuits of a comparative example, which will bedescribed later, are shunt-connected to the power amplifier. The resultof the present invention (graph 301) shown in FIG. 2 is obtained in thefollowing condition in the ranges of the above circuit constants.Namely, the first second-order series resonant circuit 108 having aseries resonance frequency of 1.6 GHz, which is twice as large as thelow first fundamental frequency F1 (800 MHz) of the fundamentalfrequency, has an inductance of 1.3 nH and a capacitance of 7.5 pF whilethe second second-order series resonant circuit 109 having a seriesresonance frequency of 1.8 GHz, which is twice as large as the highsecond fundamental frequency F2 (900 MHz) of the fundamental frequency,has an inductance of 0.6 nH and a capacitance of 12 pF.

As shown in FIG. 2, in the series resonant circuits of the presentinvention, the band-pass characteristic at the second-order harmonic(1.6 GHz) of the first fundamental frequency F1 (800 MHz) isapproximately −46 dB while the band-pass characteristic at thesecond-order harmonic (1.8 GHz) of the second fundamental frequency F2(900 MHz) is approximately −71 dB, which are excellent band-passcharacteristics that exhibit a sufficient effect of suppressing therespective harmonic frequencies. A band-pass characteristic ofapproximately −2.6 dB loss is observed at the first fundamentalfrequency F1 (800 MHz), which is ignorable.

As described above, in the present embodiment, the first second-orderseries resonant circuit 108, that is, the first second-order harmonictrap circuit is designed to have a resonance frequency matchedcorrespondingly to the low frequency (lower limit) F1 of the broadbandfundamental frequency and is composed of a combination of a largeinductor and a small capacitor, thereby suppressing the second-orderharmonic while reducing the resonance width to prevent influence on thefundamental frequency component. On the other hand, the secondsecond-order series resonant circuit 109, that is, the secondsecond-order harmonic trap circuit is designed to have a resonancefrequency matched correspondingly to the radio frequency (upper limit)F2 of the broadband fundamental frequency and is composed of acombination of a small inductor and a large capacitor, therebysuppressing loss of the band-pass characteristic of the broadbandsecond-order harmonic including the second-order harmonic of the lowfrequency F1. With the second second-order harmonic trap circuit, ofwhich resonance frequency is matched to the second-order harmonic of theradio frequency F2 of the fundamental frequency, the fundamentalfrequency component of the broadband fundamental frequency is lessinfluenced when compared with the resonance frequency matched to thesecond-order harmonic of the low frequency F1.

Hence, in the present embodiment, the combination of the firstsecond-order series resonant circuit 108 and the second second-orderseries resonant circuit 109 enables impedance control of the broadbandsecond-order harmonic. In other words, impedance control of thebroadband second-order harmonic is achieved with a single outputmatching circuit, attaining a highly-efficient and low-cost poweramplifier with a simple construction.

COMPARATIVE EXAMPLE

Power transistors in accordance with the comparative example will bedescribed next with reference to the drawings.

FIG. 3 is a circuit diagram of a power amplifier in accordance with thecomparative example, which includes a power amplifier transistor and anoutput matching circuit. As shown in FIG. 3, bias voltage is supplied toan output stage transistor 202 from a voltage source 203 via a bias lineinductor 204. A bypass capacitor 211 is provided between the voltagesource 203 and the bias line inductor 204. On an output line from theoutput stage transistor 202 to an output terminal 201 of the poweramplifier, there are provided a series inductor (output line inductor)205, an output matching shunt capacitor 206, and an output matchingseries capacitor 207. Further, in the comparative example, twosecond-order harmonic trap circuits (second-order harmonic resonantcircuits) are provided. Specifically, a first second-order seriesresonant circuit 208 in which an inductor 208L and a capacitor 208C areconnected in series to each other is provided on a line branching fromthe output line between the output end of the output stage transistor202 and the series inductor 205. A second second-order series resonantcircuit 209 in which an inductor 209L and a capacitor 209C are connectedin series to each other is provided on a line branching from the outputline between the branch point of the first second-order resonant circuit208 and the series inductor 205. An inductor 210 is provided between thebranch point of the first second-order series resonant circuit 208 andthe branch point of the second second-order series resonant circuit 209on the output line.

Referring to FIG. 2, the graph 302 indicates a band-pass characteristicobtained in the case where both the two second-order series resonantcircuits 208, 209 have an inductance of 1nH or larger (first comparativeexample), more specifically, where the first second-order seriesresonant circuit 208 has an inductance of 1.3 nH and a capacitance of7.5 pF and the second second-order series resonant circuit 209 has aninductance of 1.3 nH and a capacitance of 5.9 pF. As well, the graph 303indicates a band-pass characteristic obtained in the case where both thetwo second-order series resonant circuits 208, 209 have an inductance of1 nH or smaller (second comparative example), more specifically, wherethe first second-order series resonant circuit 208 has an inductance of0.65 nH and a capacitance of 15 pF and the second second-order seriesresonant circuit 209 has an inductance of 0.65 nH and a capacitance of12 pF.

The graph 302 in FIG. 2 proves that: in a power amplifier including thetwo second-order series resonant circuits having an inductance of 1 nHor lager, while the band-pass characteristic loss at the low fundamentalfrequency F1 (800 MHz) is approximately −2.4 dB, which involves noproblem, the band-pass characteristic losses at the second-orderharmonic (1.6 GHz) of the fundamental frequency F1 and the second-orderharmonic (1.8 GHz) of the high fundamental frequency F2 (900 MHz) areapproximately −41 dB and approximately −57 dB, respectively, whichresults in insufficient suppression of the second-order harmonic. It isalso understood that the frequency component between 1.6 GHz and 1.8 GHzis suppressed insufficiently due to the narrow resonant width.

As well, the graph 303 in FIG. 2 proves that: in a power amplifierincluding the two second-order series resonant circuit having aninductance of 1 nH or lower, while the band-pass characteristics of thesecond-order harmonic (1.6 GHz) of the low fundamental frequency F1 andthe second-order harmonic (1.8 GHz) of the high fundamental frequency F2are approximately −62 dB and approximately −79 dB, respectively, whichresults in sufficient suppression of the second-order harmonic, theband-pass characteristic at the lower fundamental frequency F1 (800 MHz)is approximately −4.1 dB, which cannot be ignored.

In contrast, the graph 301 (the present invention) in FIG. 2 provesthat: in the output matching circuit of the present invention providedwith one second-order series resonant circuit having an inductance of 1nH or larger and one second-order series resonant circuit having aninductance of 1 nH or lower, the fundamental frequency component loss isprevented while the broadband second-order harmonic component issuppressed, as described above.

1. A power amplifier for amplifying an input signal having a fundamentalfrequency of which band width ranges between a first fundamentalfrequency F1 and a second fundamental frequency F2, comprising: a poweramplifier transistor for amplifying the input signal; and an outputmatching circuit for suppressing a harmonic component included in anoutput signal from the power amplifier transistor, wherein the outputmatching circuit includes: a first series resonant circuit including afirst inductor and a first capacitor, which are connected in series toeach other between an output end of the power amplifier transistor and aground, and having a second-order harmonic frequency of the firstfundamental frequency F 1 as a resonance frequency thereof; and a secondseries resonant circuit including a second inductor and a secondcapacitor, which are connected in series to each other between theoutput end of the power amplifier transistor and the ground, and havinga second-order harmonic frequency of the second fundamental frequency F2as a resonance frequency thereof.
 2. The power amplifier of claim 1,wherein the second inductor is composed of a microstrip line.
 3. Thepower amplifier of claim 1, wherein the first capacitor is integrated ona semiconductor substrate on which the power amplifier transistor isintegrated, and the second capacitor is a chip component.
 4. The poweramplifier of claim 1, wherein the first capacitor is integrated on asemiconductor substrate on which the power amplifier transistor isintegrated, and the second capacitor is integrated on anothersemiconductor substrate different from the semiconductor substrate onwhich the power amplifier transistor is integrated.
 5. The poweramplifier of claim 1, wherein the band width is 50 MHz or larger.
 6. Thepower amplifier of claim 1, wherein the first inductor has an inductanceof 1 nH or larger.
 7. The power amplifier of claim 1, wherein the secondinductor has an inductance of 1 nH or lower.
 8. The power amplifier ofclaim 1, wherein the second inductor is arranged between the output endof the power amplifier transistor and a predetermined branch point on abias line connected to the output end and supplying power to the poweramplifier transistor.
 9. The power amplifier of claim 8, wherein thesecond capacitor is connected at one end thereof to a line branchingfrom the bias line at the predetermined branch point and is connected atanother end thereof to the ground.
 10. The power amplifier of claim 8,further comprising: a bypass capacitor having a capacitance of 1000 pFor larger and connected to a voltage source connection part of the biasline, and a part of the bias line which ranges from the bypass capacitorto the predetermined branch point has an inductance corresponding to anelectric length in a range between λ/16 and λ/8, both inclusive, whereinλ represents a wavelength of the input signal.